1. Field of the Invention
This invention relates to polymerization of lignosulfonate materials to produce fluid, high molecular weight, low viscosity products. More particularly, this invention relates to a process whereby lignosulfonate materials are initially methylolated under alkaline conditions and subsequently subjected to elevated temperature under acidic conditions to effect polymerization.
2. The Prior Art
Lignin, as it occurs in all woody plants, is a three-dimensional macromolecule consisting of rather hydrophobic phenylpropane units. In pulping processes for the manufacture of wood pulps, the lignin is chemically depolymerized and solubilized in pulping liquors to facilitate its removal from wood fibers. Consequently, industrial lignins isolated from the pulping liquors have molecular weights that are polydisperse and relatively low in comparison with the molecular weight of lignin as it occurs naturally in wood.
The relatively low molecular weight of industrial lignin such as lignosulfonates as compared with many commercially available synthetic polymers is a significant limitation which hinders the use of such materials in many application areas. For this reason, numerous methods have been proposed in the prior art for polymerization of lignins to increase their molecular weights. Thus, in U.S. Pat. No. 3,138,555 issued June 23, 1964 to King and Adolphson, an acid polymerization process is disclosed for condensation of lignosulfonate components of spent sulfite liquor-derived solids, under controlled conditions, thereby increasing the effectiveness of lignosulfonates as dispersants in clay-water suspensions, such as drilling muds. In U.S. Pat. No. 3,864,276 issued Feb. 4, 1975 to Benko and Daneault, useful dispersing agents for dyes are produced by condensation of water-soluble lignosulfonate with water-insoluble kraft lignin. In U.S. Pat. No. 3,857,830, issued Dec. 31, 1974, Briggs discloses a process for cross-linking lignosulfonates with di-epoxides to increase surface active properties and thus provide an improved flocculating agent. The process uses cross-linking agents such as di-epoxides having molecular weights in the range of from 120 to 1,800 and involves cross-linking to the extent that the phenolic content of the lignin is decreased by 40% to 95%. Ludwig, in U.S. Pat. No. 3,850,799, issued Nov. 26, 1974, describes a process whereby lignosulfonate is treated with a polyoxyalkylene di-ester of a monosulfonic acid or a polyoxyalkylene dihalide to obtain high molecular weight compositions which are effective as thickening and flocculating agents.
Known methods for polymerizing lignosulfonate may be classified into two types in terms of the mechanism by which cross-linking occurs:
(1) cross-linking or bridging of lignin molecules by reaction of free phenolic hydroxyl groups with bifunctional or trifunctional chemicals (hereinafter termed phenolic polymerization); and PA0 (2) condensing of lignin units at other non-phenolic sites (hereinafter termed non-phenolic polymerization).
In phenolic polymerization, lignin units are cross-linked, for example, by blocking the free phenolic hydroxyl groups with halogen- or epoxide-containing chemicals such as epichlorohydrin, di-epoxide and polyoxyalkylene dihalide. This type of polymerization reaction however has its accompanying disadvantages. Most notably it requires expensive cross-linking chemicals. Additionally, blocking of phenolic hydroxyl groups reduces the solubility of lignin and thus tends to increase the viscosity of reaction media. Moreover, lignin becomes deactivated via consumption of free phenolic hydroxyl groups making it unsuitable for use in many modification reactions which need free phenolic hydroxyl groups.
The disadvantages of phenolic polymerization are not present to the same extent in non-phenolic polymerization since the latter reaction mechanism is such that free-phenolic hydroxyl groups are not consumed and the lignin is not deactivated. In this reactive scheme, alkyl linkages are formed that are more stable at high temperature than the ether bonds formed via phenolic polymerization.
Non-phenolic polymerization methods for increasing the molecular weight of lignosulfonate materials have included (1) treatment with formaldehyde and kraft lignin at alkaline pH; (2) acid polymerization; and (3) treatment with formaldehyde at acidic pH.
Under alkaline conditions, lignosulfonates are condensable with non-sulfonated lignin such as kraft lignin. This is disclosed in the aforementioned U.S. Pat. No. 3,864,276. However, neither treatment of lignosulfonates in the absence of kraft lignin by heating to elevated temperature nor treatment of such lignosulfonates with formaldehyde results in condensation or polymerization to higher molecular weights when performed under alkaline conditions.
Acid polymerization, as exemplified by U.S. Pat. No. 4,100,155 issued July 11, 1978 to Ashborn and Laine and King et al U.S. Pat. No. 3,138,355 referred to above, requires heating lignosulfonate solution at acidic pH for a period sufficient to form products having a viscosity at least 25 percent higher than the viscosity of the starting material. In such reactions, selection of solids concentration is a critical factor since too low a concentration tends to hydrolyze the lignosulfonate rather than polymerize it. On the other hand, too high a concentration invariably results in thickening of the reaction solution.
Treatment of lignosulfonates with formaldehyde under acidic conditions invariably results in a substantial viscosity build-up or even gelling of the process solutions at the normal process solution concentrations of 20 to 40 percent solids. Upon gelling, the lignosulfonates are no longer cross-linkable and are thereby rendered unsuitable for use in many of the applications where higher molecular weight polymers may be used. Viscosity build-up and gelling are thus factors that severely limit the degree of polymerization that can be achieved with such non-phenolic polymerization methods.
A major object of this invention is to provide an effective method for obtaining high molecular weight lignosulfonates.
An additional object of this invention is to provide a simple process for the polymerization of lignosulfonates using formaldehyde as the cross-linking agent.
A further object of this invention is to provide a method for polymerization of lignosulfonates via a non-phenolic mechanism with minimal viscosity build-up of the reaction solutions.